Construction of bacterial expression plasmids containing envelope and pX-LOR gene sequences of the human T-cell leukemia virus type-I (HTLV-I) and env-LOR sequences of HTLV type III (HTLV-III) has been accomplished. These viral gene fragments were linked to the lambdacII gene of the high level expression vector, pJL6, or its derivatives, pJLA16 and pCJX. Two such expression plasmids, pKS400, which expresses a 16,000 dalton p21E transmembrane fusion-protein from the transmembrane domain of the HTLV-I env protein, and pKL296C, which produces a fusion-protein representing amino acid sequences from the carboxyl-terminus of the p42LOR protein of the HTLV-I pX gene, have been isolated and characterized, and their fusion-proteins purified to homogeneity. Current and projected studies with these and other expression plasmid constructs will be discussed.